Huawei wants to continue to sell smartphones in the US

What I do not understand:
1. It is not fair that Chinese brands are excluded, while American companies are just as bad and also have a lot of influence outside their country borders. Worse, I think we outsiders have absolutely no idea what happens and can not see what is done with our data / trust.

Politics, nothing more, no less. If you thought you could do an FB update in China or check your Gmail, you will not succeed. Do not think that you can send an angry tweet, that does not work either.
In short, China probably forbids more American products than the US currently does.

2. Why do Chinese companies not throw their data on the street completely? M.a.w. make your products / software open source, ensure openness and show that you are the better choice. Are they not so confident after all? Do their products, like the American ones, also consist of backdoors? Or is that simply not possible?

This is starting to happen now. And yes, backdoors. When is a backdoor backdoor and when a sloridge error in the software. I have worked for a large Chinese network camera producer. This was accused, especially from the US, of backdoors. Which I, in view of my knowledge of the dev team, dare to classify as carelessness / vulnerability.

3. Why does not a single European company enter this market? Are we really so small to compete with China / America? Why do we use Huawai for example? Are there no alternatives? We also have the expertise or is it not in our hands?

Europe has largely moved its manufacturing industry, except for England and Germany, to the Far East, but it is not much different in the US. With the exception of the defense market and the automotive industry and many related producers.
Why Huawei is used? Primary; price! I have heard from people who worked at Huawei (networking) that scored their projects and were 30-50% cheaper than the established names, such as Cisco, HP, etc.
Secondarily, the Chinese are not just copying nature and there are various innovations. I know that Huawei managed to produce greater distances and more bandwidth over fiber, making it even cheaper than the competition.